Method and apparatus for the determination of azimuth error

ABSTRACT

Azimuth error between a write head (16) and a read head (34) is determined by passing a multi-track tape (2) past the write head (16) and subsequently the read head (34). The signals written by the write head (16) and read by the read head (34) are analysed to determine the timing error (38) between best-match time-delayed portions (20-28) of these signals. This timing error (38) is indicative of the azimuth error.

The present invention relates to the determination of azimuth errorbetween determination as may be utilised in audio cassette recording andplayback.

It is known that azimuth accuracy is a fundamental feature of good audiocassette duplication. By azimuth is meant the angle between the gap ofthe read head relative to the direction of movement of the tape and alsothe gap of the write head relative to the direction of movement of thetape. The write head having supplied information to the tape. If thesetwo angles are both zero, the azimuths are zero and optimumhigh-frequency performance may be achieved. Should either azimuthdeviate from zero, then deterioration in this high-frequency range mayresult.

Conventionally azimuth accuracy is achieved by maintaining the writehead gap, during recording, at exactly 90° to the direction of tapemovement and also maintaining the read head at the same angle duringplayback. U.S. Pat. No. 4,709,288 discloses such a mechanism. A fineadjustment mechanism, such as a micrometer screw is provided by whicheither the cassette tape guide elements or the write or read head itselfmay be partially rotated or pivoted in order to maintain the head normalto the direction of the tape travel.

The above system, whilst adequate, suffers from the shortcomings that inorder for accurate micrometer calibration to be achieved, a test tapemust be used. This tape bears a recording of a sine wave at a frequencytowards the upper end of the bandwidth over which the machine is tooperate. The micrometer screw is then adjusted until the maximum signaloutput level is achieved.

It will be appreciated that to use such a test tape prior to eachplayback is, at best, a nuisance in, for example, mass reproductionsituations.

It is, therefore, an attractive proposition to have a system for themeasurement of azimuth error which does not necessarily require the useof a test tape. Furthermore it is an object of the present invention toprovide a method of measurement of the azimuth error by utilising thesignals actually recorded on the tape--rather than necessarily using adedicated test tape.

Thus according to a first aspect of the present invention there isprovided a method for the determination of azimuth error between writeand read heads moving relative to first and further signals borne by acarder comprising:

analysing the first and further signals as supplied by the write headand analysing the same signals as received by the read head, whereinanalysing of the write head and read head signals includes temporallydelaying the first signals with respect to the further signals, passingthe further signals through a plurality of time delays thereby toprovide a plurality of time-delayed further signals each having adifferent associated time delay, and providing time-delay valuescorresponding to the best correlation between the time-delayed firstsignals and each of the plurality of time-delayed further signals;

subtracting the time-delay value of the best-correlation read headsignal from the time-delay value of the best-correlation write headsignal to provide a timing error value from which the azimuth error maybe derived.

Thus the invention provides the advantage that a dedicated test tape isno longer necessary as the signals recorded on the carder by the writehead may themselves be analysed--as read by the read head--in order toprovide a timing error which directly relates to the azimuth error. Useof a test tape is, however, not precluded.

Preferably the write and read heads move at speed, v, relative to thecarrier, and the first and further signals are spaced apart on thecarder by distance, d; and wherein the azimuth error is derived bymultiplying the timing error by v/d.

Additionally the time delays may be cascaded, or in parallel.

According to a second aspect of the present invention there is providedapparatus for determination or azimuth error between write and readheads moving relative to first and further signals borne by a carder,including;

first time-delay means for delaying the first signals with respect tothe further signals, both first and further signals supplied by both thewrite head and the read head;

further time-delaying means for delaying the further signals from thewrite and read heads respectively to provide a plurality of time-delayedfurther signals each having a different associated time delay;

correlation means for providing time-delay values corresponding to thebest correlation between the time-delayed first signals and each of theplurality of time-delayed further signals from both the write head andread head;

and means for subtracting the time-delay value of the best-correlatedread head signal from the time-delay value of the best-correlated writehead signal to provide a timing error value from which the azimuth errormay be derived.

The invention will now be described, by way of example only and withreference to the accompanying drawings of which:

FIGS. 1 and 2 illustrate schematically a known form of azimuth errordetermination; FIG. 3 illustrates schematically an embodiment of thepresent invention;

FIG. 4 illustrates graphically how signals are interpolated; and,

FIG. 5 illustrates an alternative format for the time delays.

Referring firstly to FIG. 1 it can be seen that with the known azimuthapparatus, a carder, shown as tape 2, bears two tracks, 4a and 4c andtwo gaps 4b and 4d adjacent each track. The tracks 4a and 4c bearmagnetically encoded information. This encoding is well known and sowill not be discussed herein. The tape 2 moves in the direction shown byarrow 6.

A magnetic read head 8 is placed over and adjacent the tape 2 and twogaps 10 in the read head 8 are arranged to lie at 90° to the direction6, and above each track 4a, 4c.

Referring now also to FIG. 2, this adjustment is more fully described.As can be seen, the track 4a bears two sine waves. Although not shown,track 4c also bears two sine waves. The sine waves are such that as thetape moves in direction 6, if, and only if, the gap 10 is normal to thedirection, then the sine waves will be in phase as seen along the lengthof the gap. It is apparent therefore, that any deviations away from thisnormal will result in a phase difference between the sine waves asviewed by the gap 10 and hence a reduction in the output level ofsignals from the read head

In practice, it may be the tape 2 itself rather than the read head 8which will become misaligned, but in any event this will result in theangle between the gap 10 and the direction 6 being slightly deviatedfrom 90°. For a standard compact cassette tape an error of only 0.2° (or12 minutes of arc) will give a high-frequency loss of 3 dB at 10 kHz;this represents a physical misalignment of about 3 mm over a tape pathlength of 1 m. An error of around 30 minutes of arc will result incomplete cancellation of the signals at around 10 kHz.

Referring now to FIG. 3, it will be seen that the present inventionutilises any signals recorded on at least two tracks. This is because,as between the writing and reading operations, any non-zero azimuth willbe manifest as a timing error between the tracks.

First signal 12 and further signal 14 are supplied to write head 16 (ofsimilar construction to that of head 8 as illustrated in FIG. 1). Thesignal 12 is for the left channel of a stereo recording and the signal14 for the fight channel. Part of the signal 12 is tapped off and fedinto a fast time-delay means, in this example two cascaded delayelements 18,20. Similarly part of the signal 14 is tapped off and fedinto a further time-delay means, here four cascaded delay elements22,24,26 and 28.

Each of the delay elements 18-28 is chosen to introduce a delay of Tinto the signal passing therethrough. Hence the fast signal 12 isdelayed by 2T.

The first signal 12, having been delayed by 2T is then split into fiveidentical portions and each portion is input into one of fivecorrelators 30. Also input to each of the correlators 30 is each of thefurther signals 14 from the node points formed by the cascaded delayelements 22,24,26 and 28.

Hence to each of the correlators 30 is input two signals--a first signal12 of the time delay 2T and a time delayed further signal 14 having oneof the delays--O, T, 2T, 3T, 4T.

The output from each correlator 30 is a time-delay value 31 whichrepresents the difference between the two inputs it receives, and thisoutput is then input to a peak position detector 32 which determineswhich time delay 22-28 provides the best correlation result with thetime-delayed first signal 12. This may, in the simplest form, be done bythe detector 32 simply selecting a time-delay value 31 corresponding toone of its five inputs which provides the largest value; alternativelythe detector 32 may fit a curve to its five input time-delay values 3land locate the position on the curve of the correlation peak byinterpolation, thereby to ascertain a value anywhere in the range -2T to+2T. The range -2T to +2T will be understood to be derived from thedifference between the delay of first signal 12 of 2T and any one of thedelays of the delays of the further signal 14 of 0-4T.

It will be apparent that if the first signal 12 and the further signal14 are the same, then the position of the correlation peak will be zero.However, as signal 12 is the left track of a stereo signal and signal 14the right track, then, for conventional stereo signals, the peak willshift away from zero dependent upon the programme content. If theprogramme is biassed toward the left, then the right track 14 will tendto be delayed relative to the left track 12 and this will be reflectedin the position of the correlation peak. Conversely, if the programme isbiassed towards the right, the correlation peak will tend to shift inthe other direction.

Thus on its own, the position of the correlation peak is notparticularly useful. However, by analysing also the same parameter forthe signals received by the read head and subtracting this from thesignals supplied to the write head 16, then the effect of the programmecontent may be cancelled out.

Referring still to FIG. 3, the read head 33 reads off the first andfurther signals 12 and 14 subsequent to their being recorded by thewrite head 16, and exactly the same procedure is carded out as has beendescribed above, hence in the lower half of FIG. 3 like components aresimilarly numbered.

The output 34 of peak detector 32 associated with the write head 16 andthe output 36 of peak detector 32 associated with the read head is thenpassed to a means for subtracting output 36 from output 34. In thepresent example the means for subtracting is subtractor 38.

The output of the subtractor 38 will thus be a timing error associatedwith the write head 16 and the read head 33, from which the azimutherror may be readily derived.

This derivation relies on the relationship between the speed of the tape2 and the distance between the tracks 4a and 4c (FIG. 1). Assuming thatthe tape speed is v and the distance between the two tracks is d, thenthe azimuth error, e, is radians is given by: ##EQU1## d and v areeither known or may be found simply. For example, a look-up table maystore feasible combinations or sensors on or adjacent the tape maydetermine v and d as the tape moves.

In any event, the constant v/d, 40 may be multiplied with the timingerror signal 38 in multiplier 42 to provide the azimuth error, e, at 44.

It will be appreciated that as the present invention utilises a timingrelationship between the signals under consideration, then at least twosuch signals (in the above example the two tracks 4a and 4c) are needed.Using more is, of course, optional. Furthermore there must be somedegree of correlation between the signals under test as supplied to thewrite head and before recording. In the above example, this criterionhas been met by using the left and fight channels of a stereo signal.This requirement will be readily apparent because without a degree ofcorrelation in the original signals, then timing errors would bemeaningless.

Since the present invention enables determination of the azimuth errorbetween the write head and the read head, then the azimuth errors of oneof these heads must already be known. Conveniently it is the read headwhose azimuth error is known as this, as has been discussed above, mayreadily be set to zero using conventional methods.

It will be understood that the write and read operations may be achievedas part of one operation or may be two totally separate operations.However, it is necessary that the record and playback signals, that inthose signals 12, 14 associated with the write head and read headrespectively, are approximately synchronised so that the correlators 30operate on approximately the same sections of these record and playbacksignals.

In the foregoing it will be understood that by correlation means ismeant both the correlation 30 and their associated peak detector element32. This is so because both the correlators 30 and the peak detectorelements 32 are necessary to provide the maximum-value signal result.

Referring to FIG. 4 illustrates graphically how the correlation means isable to fit a curve to the data supplied thereto and locate the positionof the correlation peak by interpolation. A indicates the maximumsupplied value and yet B indicates the actual maximum value obtained byinterpolation.

FIG. 5 illustrates schematically a parallel arrangement for the timedelays 22,24,26 and 28.

It will be apparent that the present invention is not limited in itsapplication to audio azimuth error determination. Indeed any suitablesignals may be applied to any suitable carder.

claim:
 1. A method for determining an azimuth error between a write headand a read head moving relative to first and further signals borne by acarrier comprising the steps of:analysing the first and further signalsas supplied by the write head including temporally delaying the firstsignal with respect to the further signal, passing the further signalthrough a plurality of time delays to thereby provide a plurality oftime-delayed further signals each having a different associated timedelay, and providing a time-delay value corresponding to the bestcorrelation between the write time-delayed first signal and each of theplurality of time-delayed further signals; analysing the first andfurther signals as supplied by the read head including temporallydelaying the first signal with respect to the further signal, passingthe further signal through a plurality of time delays to thereby providea plurality of time-delayed further signals each having a differentassociated time delay and providing a time delay value corresponding tothe best correlation between the read time-delayed first signal and eachof the plurality of time-delayed further signals; and subtracting thetime-delayed value of a best correlation read head signal from thetime-delay value of a best correlation write head signal to provide atiming error value from which the azimuth error may be derived.
 2. Amethod according to claim 1 and further comprising the steps of movingthe write and read heads at speed, v, relative to the carrier, spacingthe first and further signals apart on the carrier by distance, d andderiving the azimuth error by multiplying the timing error by v/d.
 3. Amethod according to claim 1 and further comprising the step of cascadingthe time delays.
 4. A method according to claim 2 and further comprisingthe step of cascading the time delays.
 5. A method according to claim 1and further comprising the step of providing the time delays inparallel.
 6. A method according to claim 2 and further comprising thestep of providing the time delays in parallel.
 7. An apparatus fordetermination of azimuth error between a write head and a read headmoving relative to first and further signals borne by a carrier,comprising:first time-delay means for delaying the first write signalwith respect to the further signal supplied by the write head; otherfirst time-delay means for delaying the first read signal with respectto the further signal supplied by the read head; further time-delaymeans for delaying the further signal from the write head to provide aplurality of time-delayed further signal each having a differentassociated time delay; other further time-delay means for delaying thefurther signal from the read head to provide a plurality of othertime-delayed further signals each having a different associated timedelay; correlation means for providing outputs corresponding to the bestcorrelation between the time-delayed first write and read signals andthe further signals supplied by the write and read heads and also theplurality of time delayed further signals from the further time delaymeans and the plurality of other time-delayed further signals from theother further time-delay means respectively; means for subtracting thetime-delay value of the best-correlated read head signal from thetime-delay value of the best-correlated write head signal to provide atiming error value from which the azimuth error may be derived.
 8. Anapparatus according to claim 7 wherein the correlation means comprises aplurality of correlators and peak-position detectors.
 9. An apparatusaccording to claim 7 wherein the means for subtracting comprises asubtractor.
 10. An apparatus according to claim 8 wherein the means forsubstracting comprises a subtractor.
 11. An apparatus according to claim7 wherein the further time-delay means comprises cascaded time delays.12. An apparatus according to claim 8 wherein the further time-delaymeans comprises cascaded time delays.
 13. An apparatus according toclaim 9 wherein the further time-delay means comprises cascaded timedelays.
 14. An apparatus according to claim 7 wherein the further timedelay means comprise a parallel arrangement of time-delays.
 15. Anapparatus according to claim 8 wherein the further time delay meanscomprise a parallel arrangement of time-delays.
 16. An apparatusaccording to claim 9 wherein the further time delay means comprise aparallel arrangement of time-delays.
 17. An apparatus according to claim7 further including a multiplier for multiplying the timing error valuewith a predetermined constant to provide the azimuth error.